Estimation of charge injection at Al/LDPE interface based on space charge and external current measurement

Space charge accumulation in polymer is a key problem for the electrical aging of insulation under DC condition. Space charge accumulation is strongly determined by the charge injection rate. The estimation of charge injection with steady state current contains the extraction current and is not accurate. This paper proposes a quantitative estimation method of the charge injection at the metal/polymer interface with space charge and current simultaneous measurement. A double-layer sample with a blocking layer is employed to make a quasi-unipolar condition to remove the extraction current. In addition, the displacement current is removed by considering the change in electric field on the electrode. The relationship between the electrode electric field and the pure injection current is analyzed. It is found that the square root of electric field intensity and the logarithm of injection current at the Al/LDPE (low density polyethylene) interface are linear with each other. The coefficients β for negative and positive charge injection at Al/LDPE interface are about 8.44 × 10⁻²⁴ and 8.32 × 10⁻²⁴, respectively. It indicates that both negative and positive charge injection at Al/LDPE interface fit the mathematical relationship of the Poole-Frenkel effect, rather than the Schottky effect which is mostly used in the study on the charge transport at polymer/metal. The negative and positive charge injection barriers are 1.23 and 1.24 eV, respectively.